Modelling stormwater runoff, quality, and pollutant loads in a large urban catchment; pp 225–242

Identification of stormwater runoff, its pollution load, and their implications for land use is essential in implementing stormwater management strategies. Hydrologic modelling provides an opportunity to assess them at limited data resources. In this study, the stormwater management model SWMM5 is applied for model development for a large basin in Tallinn. A geographic information system tool is used for subcatchment delineation, identification of directly connected impervious areas (DCIAs), and preparation of catchment input parameters. The model is calibrated and verified using sampled storm events to estimate event mean concentrations and annual loads. The predictive capability of the model for quantity is good and for quality moderate. The findings from the model show the percentage of the impervious area in the large catchment to be low at 19.7%. Although DCIAs, in particular roads and roofs, have relatively smaller areas they significantly impact runoff production (up to 75%) and loads (up to 66% total phosphorus and 71% total suspended solids). The first flush at the beginning of runoff is less important in case of a low intensity of rainfall, but heavy rain and snowmelt generate substantial runoff and pollution loads. When grab sampling is applied, it should focus on the medium and large events within 6 hours of storm commencement in order to achieve better mass estimations.